1. Field of the Invention
The present disclosure relates to active pixel sensors, and more particularly relates to metal-oxide semiconductor active pixel sensors.
2. Description of the Related Art
Active pixel sensors (APS) convert photo images into electric signals, and are widely used in digital cameras, mobile phones with built-in cameras, monitoring systems, and the like.
Presently, active pixel sensors are roughly classified as a type of charge coupled device (CCD) and a type of complementary metal-oxide semiconductor (CMOS). The CCD type generally has lower noise levels and better image quality than the CMOS type, but has disadvantages in manufacturing costs and power consumption. The APS of the CMOS type can be manufactured by common semiconductor manufacturing technologies, and easily integrated into other systems such as amplifiers or signal processing units. The CMOS type also has high processing speeds and much lower power consumption than the CCD type.
The CMOS type, however, is disadvantageous in terms of noise and image quality, due to a low signal-to-noise ratio (SNR) and a narrow signal dynamic range.
A one-transistor structure and a three-transistor structure were used in the past for manufacturing the CMOS active pixel sensors, but as of recently, a four-transistor structure is being commonly used for manufacturing the CMOS active pixel sensors. In the four-transistor structure, one pixel of the CMOS sensors is composed of a photodiode and four MOS transistors. Photo-generated charges integrated in the photodiode are transferred under the control of the four transistors.
FIG. 1 is a cross-sectional view of a four-transistor CMOS active pixel sensor according to a generally conventional example. Referring to FIG. 1, a four-transistor CMOS sensor 100 includes a photodiode area 110, and N-type regions 120 and 130 formed upon a P-type silicon substrate 101. An insulation layer 140 is formed over the photodiode area 110 and the N-type areas 120 and 130. For example, the insulation layer 140 may include a SiO2 layer.
A transfer gate electrode TG and a reset gate electrode RG are formed on the insulation layer 140. A transfer gate control line TGC and a reset gate control line RGC are respectively connected to the transfer gate electrode TG and the reset gate electrode RG.
The N-type area 120 acts as a floating diffusion node. The pixel 100 further includes a source follower transistor SF for detecting signals from the N-type area 120, as well as a selection transistor SEL to be turned on in response to a selection signal of a selection signal line SELC.
The photodiode area 110 appears to occupy a narrow area in the cross-sectional view, but actually occupies an area larger than that of the N-type areas 120 and 130, so as to generate photoelectrons. A shield layer, not shown in FIG. 1, is formed over areas other than the photodiode area 110 so as to prevent light from being incident on the other areas.
MOS transistors 150 and 160 are respectively formed by the N-type areas 120 and 130 along with the electrodes TG and RG formed on the insulation layer 140 of the silicon substrate.
The transfer transistor 150 is controlled by a transfer control signal of the transfer gate control line TGC connected to the transfer gate electrode TG, and transfers photoelectrons integrated within the photodiode area 110 to the floating diffusion node 120. The reset transistor 160 is controlled by a reset control signal of the reset gate control line RGC connected to the reset gate electrode RG, and resets an initial potential of the floating diffusion node 120.
The source follower transistor SF detects a potential variation of the floating diffusion node 120, and transfers the detected potential of the floating diffusion node 120 to internal circuits (not shown) in the next stage of the CMOS active pixel sensor under the control of the selection transistor SEL. The internal circuits may include an amplifier, a sampling circuit for sampling the transferred signal, and the like.
Operations of the active pixel sensor in FIG. 1 are performed as follows. When a voltage level of the reset gate electrode RG is raised by the reset gate control line RGC and the reset transistor 160 is turned on, a voltage level of the floating diffusion node 120 increases up to a power supply voltage VDD. The source follower transistor SF and the selection transistor SEL perform a first sampling of a potential of the floating diffusion node 120, and the potential is referred to as a reference potential.
While external light is incident onto the photodiode areas 110 during a photo integration period, electron-hole pairs are generated in proportion to the amount of the incident light. Next, when voltage of the transfer gate electrode TG is raised by the transfer control signal of the transfer gate control line TGC, a channel is formed beneath the transfer gate electrode TG and electrons integrated within the photodiode area 110 are transferred to the floating diffusion node 120. The potential of the floating diffusion node 120 drops in proportion to the amount of the transferred electrons, and then a potential of the source of the source follower transistor SF is altered.
Finally, the selection transistor SEL is turned on, and the potential of the floating diffusion node 120 is transferred through the source follower transistor SF. A photo sensing is completed by obtaining a difference between the reference potential and the detected potential of the floating diffusion node 120 (which is referred to as a correlated double sampling). Then, the operations are repeated from the reset operation.
As illustrated above, the active pixel sensor 100 senses signals based on the potential variation of the floating diffusion node. This is a potential difference between an initial potential, which is raised up to a given level, of the floating diffusion node 120 of the active pixel sensor 100 and a sampled potential lowered due to the amount of electrons transferred from the photodiode.
Therefore, a range of the potential difference of the floating diffusion node 120 of the active pixel sensor 100 represents a dynamic range of light signals sensed by the active pixel sensor 100, and it is advantageous to raise the initial potential of the floating diffusion node 120 up to as high a level as possible, and then to lower the initial potential of the floating diffusion node 120. However, when a power supply having a limited voltage level is used, it is difficult to raise the initial potential of the floating diffusion node 120 beyond a given potential level.
Failure to empty all photoelectrons within the photodiode at one sensing operation causes an image lag. To empty all photoelectrons in one sensing operation, the higher potential of the transfer gate electrode is preferred. However, when power supplies having limited voltage levels are used, it is still difficult to raise the initial potential of the floating diffusion node 120 beyond the given potential level.